Answer

The length of the shadow is 13.4 feet

Work Step by Step

Let $h$ be the person's height. We can use $h$ and $\theta$ to find the length $L$ of the shadow:
$\frac{h}{L} = tan(\theta)$
$L = \frac{h}{tan~\theta}$
$L = \frac{5.75~ft}{tan~(23.4^{\circ})}$
$L = 13.4~ft$
The length of the shadow is 13.4 feet